Search Results (7 total)

The depth distribution function (DDF) ϕ(z) in normal photoemission from solids is studied by use of quantum full multiple scattering theory and non-Hermitian optical potential Σ. The damping of the photoelectron waves is taken into account without use of ad hoc assumption: Re  Σ has influence on the elastic scatterings from surrounding atoms and the Im  Σ on the damping of the photoelectron wave. The present theoretical approach explicitly takes the details of atomic arrangement in solids into account, which is in contrast to the currently used classical approaches applied to jellium models. The latter approach cannot study interference effects caused by elastic scatterings from different atomic sites. To properly describe the DDF, full multiple scattering renormalization is inevitable even at 1000  eV, which needs large scale computations. The elastic scatterings give rise to a peak at z∼4.1  Å in the DDF both for one- and three-dimensional models. Temperature effects on the DDF are also discussed, which smear the DDF because the quantum interference effects are destroyed because of the thermal motions. The asymptotic behavior of ϕ(z) for large z shows simple exponential decay from which we can estimate “mean free path.” It is, however, different from the original one, since elastic scatterings are renormalized to it.

Single (on-site) and multisite (multiatom) resonant photoemission (MARPE) processes are systematically studied by use of nonrelativistic Keldysh Green’s function theory. We apply skeleton expansion in terms of renormalized one-electron Green’s functions. In this theoretical framework we discuss the importance of the radiation field screening and the dynamically polarized part W_p in the screened Coulomb propagator. The radiation field screening plays a crucial role in observing the MARPE: We obtain expressions of the resonant processes (on site and multiatom) in terms of an x-ray absorption factor whose imaginary part is proportional to the x-ray absorption intensity. If we neglect W_p, the calculated MARPE intensity is much smaller than the observed one. We also point out the importance of the structure factor in the MARPE analyses. Typically highly symmetric atomic arrangement around a photoemitting atom provides us with no MARPE signal, but outermost oxygen atoms give rise to considerably strong MARPE because of symmetry lowering. On the other hand, low symmetric systems like rutile, perovskite, and α-alumina (Fe₂O₃) structures can give rise to finite MARPE even in the case of photoemission from inner layers of perfect crystals. The polarization dependence of the MARPE follows the same selection rule as the main photoemission processes, whereas the O 1s MARPE from Fe₂O₃ shows a rather complicated rule. These specific features of MARPE provide useful local structural information.

Resonant emission spectra of YF₃, YCl₃, Y₂O₃, and Y-metal are observed across Y L_III absorption threshold. When the incident photon energy is below the absorption threshold, only Raman scattering component is observed. The Raman-scattering component has a two-peak structure, main and subpeaks. Above the absorption threshold, both Raman and normal L_α emission peaks are observed in insulator samples. The intensities of the Raman peak are resonantly enhanced as the incident photon energy reaches the absorption edge. On the other hand, in Y metal, only normal L_α emission peak is observed. Obtained spectra are decomposed into Raman and normal L_α emission peaks by line-shape analysis. The results are compared with theoretically calculated ones and are discussed the relaxation of core-electron excitation processes. The onset energy of the normal L_α emission peak is estimated and discussed in relation of the electronic structure of these compounds.

We have performed an x-ray photoemission (XPS) experiment using tunable x rays in the photon-energy range between 1500 and 3000 eV. The deep metal 3p core-level spectra of the tetravalent oxides MO₂, M=Ce, Pr, Tb, and Hf have been measured. The results show that the configuration interaction between localized 4fⁿ and 4fⁿ⁺¹L configuration in the initial state gives final-state peaks 3p4fⁿ, 3p4fⁿ⁺¹L, and 3p4fⁿ⁺²L². The final states of Ce 3p core levels of CeO₂ have been calculated by the many-body theory in the frame of the filled-band Anderson model.

Photoemission measurements have been performed on the high-T_c superconductor YBa₂Cu₃O_6.85 using synchrotron radiation of 20- 110 eV. It is found that the valence band is shifted by 1.1 to 1.6 eV towards higher binding energy relative to band calculations. The effective intraatomic Coulomb energy between d electrons (U_dd) was evaluated from the energy position of the valence-band satellite due to the two-hole bound state: U_dd=5-6 eV, which is comparable to the valence-band width (W=6-7 eV). This strongly suggests that electron correlation plays a key role in characterizing the electronic properties of this oxide superconductor.

Continuum x rays induced by bombardments of a Be target with 20.14-MeV/amu protons and ³He²⁺ ions have been measured with a Si(Li) detector in the direction of 90° with respect to the incident beam. Differences in the x-ray-production cross sections multiplied by the x-ray energy ℏ ω {ℏ ω[σ_h(ℏ ω) / 4-σ_p(ℏ ω)]} and ratio of the cross sections R (ℏ ω) [≡σ_h(ℏ ω) / 4σ_p(ℏ ω)] for the proton and ³He²⁺-ion impact, where σ_h(ℏ ω) and σ_p(ℏ ω) are the x-ray-production cross sections for proton and ³He²⁺-ion impact, respectively, were obtained as a function of the x-ray energy. Both the difference and the ratio show peaks in the region where the x-ray energy is equal to the relative kinetic energy between the projectile and the inner-shell electron to be scattered by the projectile. From the comparison with a theory of quasi-free-electron bremsstrahlung based on the impulse approximation, it is found that this peak corresponds to the maximum of the velocity-distribution function of the inner-shell electron. Furthermore, the contribution of the radiative electron-capture process in the case of proton and ³He²⁺ impact is clearly found.

Ratios of ionization cross sections for proton impact to those for ³He-ion impact have been measured on Al K and Y L shells at projectile energies of 7, 9, 12, 16, and 21 MeV /amu. The ratio tends to unity with an increase in the projectile energy for both the Al K and Y L ionizations. This behavior can well be explained in terms of the Binstock-Reading theory combined with the distant-collision theory of Hill and Merzbacher: The polarization effect in distant collisions and the decreasing electron-density effect in close collisions cancel out each other in this high-energy region.